Amygdalo-cortical
Development and the Emergence of Primate Social Behavior

A number of studies in
non-human
primates have suggested that the amygdala is essential for the normal
production and interpretation of social signals. Network researchers
conducted a study in rhesus monkeys that challenges this long-held
belief. Adult monkeys with amygdala lesions appear to demonstrate
normal social behavior, while infant monkeys with amygdala lesions in
previous studies have demonstrated impairments in social behavior.
However, these abnormalities may not have been attributable to the
absence of the amygdala, but may rather have been attributable to
abnormal rearing conditions (isolation or peer-rearing) or to
additional damage to structures surrounding the amygdala due to less
sophisticated lesioning methods than are currently available. This
study found that infant rhesus monkeys with very precise lesions of the
amygdala who were reared with their mothers did develop
species-appropriate social behaviors, indicating that the amygdala is
not necessary for normal social development. However, these animals did
produce more inappropriate fear behaviors (that is, fear behaviors that
were not in response to behaviors that would be expected to elicit
fear, such as aggression) during social encounters than normal animals
or animals with hippocampal lesions. The animals with amygdala lesions
also showed heightened affiliative behaviors. The production of more
fear behaviors and more affiliative behaviors by the same animals may
be due to hyper-responsivity or hyper-vigilance in the lesioned
animals, leading to an over-production of all social signals in an
attempt to appease other animals. It may also be due to an inability to
interpret social signals from other animals.

This research is
important for a number of reasons. First, previous research on
amygdala-lesioned animals was used as the basis for a proposed
“animal model” of autism due to the autistic-like
behavior of the lesioned animals. This model proposed that dysfunction
of the amygdala may have been responsible for some of the
characteristics of autism. However, as this study found that the
presence of a functional amygdala early in development is not necessary
for the development of appropriate social behavior, it challenges the
involvement of the amygdala in autism. It does, however, suggest that
the amygdala may be involved in anxiety disorders. Secondly, in
demonstrating that the amygdala is not necessary for the development of
normal social development, it necessarily calls into question the role
of other structures in such development.